The present invention is in the field of interference mitigation in wireless networks, as for example in cellular mobile communication networks.
Recently, much research has been done in the area of mitigating or avoiding interference in TDD (Time Division Duplex) networks by means of power control, cf. A. Tyrrell, H. Haas, G. Auer, and P. Omiyi, “Decentralized Interference Avoidance Using Busy Bursts,” in Proc. of IEEE International Conference on Communication (ICC 2007), Glasgow, UK: IEEE, Jun. 24-28, 2007 and G. Auer, A. Tyrrell, and H. Hass, “Decentralized C/I Power Control for TDD,” in Proc. of the IEEE Vehicular Technology Conference (VTC), Marina Bay, Singapore: IEEE, May 11-14, 2008. However, with increasing interest in LTE-A (Long Term Evolution-Advanced), TDD systems are migrating towards the adoption of OFDMA (Orthogonal Frequency Division Multiple Access).
As LTE-A also operates in FDD (Frequency Division Duplex) mode, where uplink and downlink are separated in different frequency bands, interference avoidance schemes applicable to FDD need to be devised. Unlike the TDD mode, FDD lacks channel reciprocity with respect to frequency selective fading, so that the interference mitigation techniques used in TDD systems are either not applicable for FDD or is to be modified in order to work properly.
UL (Uplink) interference is an area of great concern for future systems. As cell sizes shrink, the amount of interference in the UL increases. In traditional cellular systems, ideally, UL interference originates only from cells other than the cell of interest. However, with the promotion of the femto-cell concept, cf. Z. Bharucha, I. Ćosović, H. Haas, and G. Auer, “Throughput Enhancement through Femto-Cell Deployment,” in Proc. of the 7th International Workshop on Multi-Carrier Systems & Solutions (MC-SS), Herrsching, Germany: IEEE, May 5-6, 2009, V. Chandrasekhar, J. Andrews, and A. Gatherer, “Femtocell Networks: A. Survey,” IEEE Communications Magazine, vol. 46, no. 9, pp. 59-67, 2008, H. Claussen, “Performance of Macro- and Co-Channel Femtocells in a Hierarchical Cell Structure,” in Proc. of the 18th IEEE International Symposium on Personal, Indoor and Mobile Radio Communications (PIMRC), Athens Greece, Sep. 3-7, 2007, pp. 1-5, H. Claussen, L. Ho, and L. Samuel, “Self-Optimization of Coverage for Femtocell Deployments,” in Proc. of the Wireless Telecommunications Symposium (WTS), Apr. 24-26, 2008, pp. 278-285, and L. Ho and H. Claussen, “Effects of User-Deployed, Co-Channel Femtocells on the Call Drop Probability in a Residential Scenario,” in Proc. of the 18th IEEE International Symposium on Personal, Indoor and Mobile Radio Communications (PIMRC), Athens, Greece, Sep. 3-7, 2007, pp. 1-5, this is not the case any more.
With the deployment of femto-cells, UL interference also comes from the macro-cell of interest since femto-cells will lie within macro-cells in an uncoordinated fashion. Maximising the capacity of such systems depends heavily on curtailing the UL interference caused at the BS (Base Station). This is dealt with in this work.
One previous proposal for interference tolerance signalling in TDD systems, cf. P. Agyapong, H. Haas, A. Tyrrell, and G. Auer, “Interference Tolerance Signaling Using TDD Busy Tone Concept,” in Proc. Of the Vehicular Technology Conference (VTC), Dublin, Ireland: IEEE, Apr. 22-25, 2007, pp. 2850-2854 may be of interest here. For an ongoing link, the maximum tolerable interference is established based on the signal-to-interference-plus-noise ratio (SINR) requirements of that link. Every potential interferer attempts to adjust its transmit power such that this level is not exceeded. Doing this establishes a “transmission region” around the BS. The lower the interference power, the larger is the transmission region.
FIGS. 12a and 12b illustrate an example of a network situation, in which interference mitigation is critical. In FIG. 12a on the left hand side there are two base stations B1 and B2. Furthermore, there are two mobile stations M1 and M2. M1 is actively communicating with B2, indicated by a solid arrow pointing from M1 to B2. While communicating M1 creates interference received at B1. Due to said interference, M2 cannot set up a connection to B1. M1 interferes with the uplink (UL) transmission of M2. From FIG. 12a it is seen that the uplink communication between B1 and M2 cannot take place because of the simultaneous and highly interfering uplink transmission between M1 and B2.
FIG. 12b illustrates the same situation, however, after M1 has reduced its transmission power due to SINR adjustments carried out or controlled by B2. After M1 having reduced its transmission power, the transmission region inflates to encompass the transmission between M2 and B1. In other words, the interferer adjusts its transmission power, thus allowing the vulnerable transmission to take place. By modulating the tolerable interference level, the size of the hearability distance, which is indicated in FIGS. 12a and 12b by the grey circle in the center of the respective cells or base stations, can be changed. From FIGS. 12a and 12b it can be seen that lowering the interference will increase the hearability distance and vice versa.
FIG. 13 illustrates the hearability distance with the help of a view graph. FIG. 13 shows the hearability distance around a base station, where the hearability distance is inversely proportional to the interfering power.
The above-described conventional concept however, has the disadvantage that uplink interference mitigation is carried out by the base stations, in terms of SINR adjustment. In case of time division duplex (TDD) systems, channel reciprocity is exploited, enabling to conclude between uplink and downlink channel coefficients. However, in a frequency division duplex (FDD) system, channel reciprocity between uplink and downlink is not present anymore.